Platinum complexes of triaminotriphenylmethanes: interaction with DNA and radiosensitization

Complexes of the negatively charged tetrachloroplatinum (II) dianion and the positively charged triaminotriphenylmethane nuclear dyes, Basic Fuchsin and Methyl Green have been prepared. Elemental analysis indicated that the complexes associate as PtCl₄(Basic Fuchsin)₂ and PtCl₄(Methyl Green) in neutral, tight ion pairs. Studies with pBR322 DNA indicated that Pt(Basic Fuchsin)₂ and Basic Fuchsin produced no irreversible DNA changes, but that Pt(Methyl Green) readily and Methyl Green less efficiently produced single strand breaks in the DNA. Cytotoxicity studies in exponentially, growing asynchronous EMT6 cells in vitro showed that Pt(Basic Fuchsin)₂ was more cytotoxic than Pt(Methyl Green) and that both complexes were more toxic to normally oxygenated than hypoxic cells and to cells at normal vs. acidotic pH (6.45). Radiation studies, also in EMT6 cells, demonstrated that Pt(Methyl Green) was an effective radiosensitizing drug in hypoxic cells (dose modifying factor [DMF] 1.9), but that neither the free dyes nor Pt(Basic Fuchsin)₂ had significant radiosensitizing activity. In vivo studies in the FSaIIC fibrosarcoma confirmed the radiosensitizing properties of Pt(Methyl Green) (DMF 2.7), and indicated that Pt(Basic Fuchsin)₂ also had some activity (DMF 1.4) which was equivalent to that of mizonidazole (DMF 1.4). These investigations suggest that these or similar platinum-dye complexes may have important antitumor applications in the human clinic.